RF oscillators, especially voltage controlled oscillators (VCOs), are essential to RF communication, broadcasting, and receiving systems, and can function to generate, track, clean, amplify, and distribute RF carriers. VCOs in a phase-locked loop configuration can also be used for clock recovery, carrier recovery, signal modulation and demodulation, and frequency synthesizing.
Photonic RF systems embed photonic technology into traditional RF system designs to employ optical waves as carriers for transporting RF signals via optical fibers to remote locations. Additionally, certain RF signal-processing functions such as signal mixing, antenna beam steering, and signal altering can also be accomplished in the optical domain. Photonic technology offers many advantages including low loss, light weight, high frequency, high security, remoting capability, and immunity to electromagnetic interference, all highly desirable in most RF systems.
Conventionally, generating a high-frequency RF signal in the optical domain has been accomplished by modulating a diode laser or an external electro-optical (E/O) modulator using a high-frequency stable electrical signal from a local oscillator (LO). Such an LO signal is generally obtained by multiplying a low-frequency reference such as a quartz oscillator to the required high frequency with several stages of multipliers and amplifiers. Consequently, the resulting system is bulky, complicated, inefficient, and costly. Another known alternative for generating photonic RF carriers entails mixing two lasers with different optical frequencies. However, the resulting bandwidth of the signal is wide (limited by the spectral width of the lasers, typically greater than tens of kilohertz) and the frequency stability of the beat signal is poor due to the drift of the optical frequency of the two lasers.
A relatively recent improvement in photonic RF systems is the photonic oscillator, which can provide very low phase noise multi-tone RF oscillations and essentially is a special VCO with both optical and electrical outputs. Fundamentally, the absorption of a light wave supplied to an end of a semiconductor optical absorption layer is controlled by changing the intensity of an electric field applied to the semiconductor optical absorption layer. With reference to FIG. 1, a conventional multi-tone photonic oscillator 10 includes a laser source 100, an optical modulator 110, one or two lightwave delay paths 130, 132 and related beam splitter 120, a photodetector 140, 142 for each delay path and optionally an RF coupler 150, a low-noise electrical amplifier 160, an RF coupler 180 to divide out the signal carrier and the feedback signal, and a RF bandpass filter 190.
Laser light emitted by the laser source 100 supplies the power for the oscillator 10 and is modulated by the feedback RF signal at the electrical input of the optical modulator 110. The modulated lightwave is then sensed by photodetectors 140, 142 whose electrical output is fed back to the modulator 110 following amplification by the amplifier 160 and bandpass filtering by the filter 190. The bandpass filter 190 thus sets the bandwidth of the generated RF multitone comb spectrum.
Photonic oscillators are finding widespread use in a variety of radar and communication applications, all of which would benefit from simpler, more compact oscillator designs. Therefore, what is needed is a method and apparatus for further simplified low phase noise carrier signal generation. The embodiments of the present disclosure answer this and other needs.